1. Field of the Invention
The present invention relates to the field of power inverters and to such an inverter for the conversion of an AC or DC supply of power to typically high frequency output power to drive gas discharged lighting sources including fluorescent tubes, HID (high intensity discharge lamps), and UV generating lamps as well as flat panel displays. The system is such that the output power is responsive to a variation in the drive frequency or pulse width. The frequency or pulse width is adjusted by a microprocessor, which in fact, is the oscillator for developing the output frequency.
2. Brief Description of the Prior Art
There are many varied public domain circuits involving the generation of high frequency inputs for driving fluorescent lamps, compact fluorescent lamps, high intensity discharge and other forms of gas discharged lighting. The general terms for such circuits is xe2x80x9cswitching invertersxe2x80x9d.
There are two basic ways of generating AC output power. The first is a conventional push-pull amplifier utilizing two transistors switching alternately to a common bus as shown in FIG. 1. The second is a full or half bridge, which normally does not require a transformer unless isolation is needed, where current is alternately switched from the supply and then to the common as shown in FIG. 2. The novelty of active patents normally involves how these transistors are switched. An example of a novel half bridge configuration is my U.S. Pat. No. 5,841,650 issued Nov. 24, 1998 which shows a resonant half bridge inverter. The frequency is determined by the resonant frequency of the inductance and capacitance values along with the impedance of the load. Such circuits work fine as long as the output is held at a pre-determined value. Thus, the components are selected to supply the appropriate output power. If it is desirable to vary the output power such as dimming a lamp, it is necessary to vary the frequency or the on time of each of the switching transistors to control the amount of energy delivered to the output.
Each type of inverter has its advantages and disadvantages. The push-pull method, often referred to as the parallel resonant inverter, when it is self oscillating, is easier to drive as both switches are connected to the common bus. However this configuration requires much higher voltage switching devices than the bridge method, in addition push-pull switching has never reached the efficiency of a bridge operation.
Bridge inverters require lower voltage switches but are harder to drive as one of the switches is not connected to the common bus. See FIG. 2. When resonance is employed, a runaway condition can occur if proper feedback is not introduced to control the frequency. The saturating core inverter drive is good for low cost designs as in a limited number of components are needed. However, it is not exceptionally tolerant of variations in loads and input voltages.
The most efficient circuit relative to its cost, is the driven half bridge with a partially or fully resonant load. The problems that have arisen with this type of circuit involve out of the ordinary conditions where the load is suddenly interrupted by lamp failure or breakage, or where a lamp will not start due to age, temperature, or other reasons. The driving circuit must constantly monitor output, current and voltage, in order to make sure that no damage occurs when any of these unexpected events occurs. Up to now this has involved a very complex amount of logic circuitry and extra expensive design of the resonant components. The cost of which has offset the savings by the higher efficiency and performance of the circuit.
Accordingly, the above problems and difficulties are obviated by the present invention which incorporates a microprocessor to determine all the logic functions in software rather than hardware. In addition, a driving oscillator maybe eliminated since the microprocessor can supply the driving frequency. Because almost any function can be specified in software, the device may become much more versatile driving many different types of loads with the same circuitry.
Three types of driving theory are employed. The first, allows a feedback to maintain the output circuitry at resonance or at some offset from resonance regardless of the frequency. This is particularly useful when the load is likely to vary a great deal in terms of its capacitance or resistance. Thus the microprocessor can maintain the output at resonance for full output or at some offset from resonance to adjust the amount of output regardless of the frequency needed to do this. Frequency will be adjusted to maintain the output components at resonance. This is accomplished through feeding back the phase angle of the AC signal, at the junction of the major series resonant components, to the microprocessor chip such that the proper phase angle can be maintained for resonant operation. Output voltage and current are also fed back in order for the chip to monitor and prevent unstable operation of the inverter should the load malfunction or be suddenly removed.
A second method of drive involves setting the frequency to control the output power based on operating on one side of a resonance curve. Here, as the output is decreased, the frequency normally will rise, and when the output is to be increased, the frequency is lowered. With this circuit the frequency is directly controlled by the feedback and not the resonant point as in the first option. Either mode of operation may be assumed simply by changing the software, thus reducing the number of hardware configurations necessary to enter a marketplace. The microprocessor further allows different options that heretofore would have been too expensive to add because of the amount of hardware required. One example would be the computation of the output power by taking, a feedback of the output voltage and the output current. The processor can multiply these two together to calculate the actual output power and adjust the frequency to maintain a predetermined amount of power regardless of the voltage or current.
A third method of adjusting the output power is by changing the on time of each of the switching transistors while maintaining the frequency at the resonant point. This is sometimes referred to as pulse width modulation. (PWM).and can be done in two ways. One way is to lower the on time of each transistor while increasing the off time to maintain the same frequency. The second way is to lower the on time of one transistor while increasing the on time of the second transistor to maintain the frequency constant.
Several methods of control are made possible by the use of a microprocessor without the addition of expensive parts. One is to connect a photocell or light measuring device to the input of the microprocessor such that the output can be adjusted to maintain a particular light level. Another is to have, the microprocessor monitor for information on the power line such as a power line carrier signal or a phase chop dimming system like a SCR wall dimmer to request different output levels, in other words, dimming the lamp. Still another control method would be to allow the local control of the light level through the use of a remote control such as the ones that control a TV set.
Therefore, it is among the primary objects of this invention to supply a simple, but extremely versatile device for generating high frequency drive for gas discharged lamps or other devices that use a higher than line frequency drive.
It is another object of this invention to reduce the amount of hardware components to a minimum to accomplish this.
Yet another object of the invention is to allow for the output to be adjusted over a wide range of parameters such as power, light level local control and system control systems.
The final object of the invention is to accomplish the above objects for a price that makes the device acceptable in the marketplace.